Electrode lifetime in general and cathode performance in particular has long been recognized as one of the most important considerations affecting the viability of a plasma process. Rapid electrode deterioration can diminish the value of a process both from an economic and a technical point of view. Consequently, parameters such as the frequency of reactor "down-time" for electrode replacement, the cost of electrodes, and the contamination of products with materials emitted from the electrodes, are crucial in determining the ultimate success of plasma technology. Of even greater importance is the stability of the plasma, which can be greatly affected by phenomena occurring on the surface of the cathode.
Early recognition of the importance of electrode phenomena has provided incentive for much research in this field during the past fifty years. Yet, despite this intensive effort, the phenomena occurring at the electrodes, and especially the cathode, remain one of the least understood areas of plasma science. Nonetheless, some theories have been developed and used with some success to explain empirically-attained data. Most of them begin by assuming a mechanism of electrode emission and assessing the state of the cathode surface during such emission. Two such theories have become dominant and have provided the scientific base for all electrode development to date. The two theories describe the mechanism of cathodic electron emission as follows:
1. For sufficiently high temperatures at the cathode surface and low field strength, the current can be carried mostly by electrons which have been thermally emitted from the cathode. This method of electron emission is commonly referred to as "thermionic emission" and is characterized by cathode surface temperatures above 3,000.degree. C. and current densities of around 10.sup.3 to 10.sup.4 A/cm.sup.2. Only refractory materials such as tungsten and carbon have high enough boiling points to allow for thermionic emission. These materials are referred to as thermionic emitters.
2. For sufficiently high field strength in front of the cathode surface, emission can occur at relatively low temperatures (below 2,700.degree. C.), with the cathode material releasing electrons whose energy is below the Fermi level. This mechanism is commonly referred to as "field emission" and is characterized by current densities higher than 10.sup.6 A/cm.sup.2. Non-refractory materials such as copper and aluminum are used in field emitting electrodes and are thus known as field emitters.
Today, most workers in the field agree that in real arcs one deals with a combination of thermionic and field emissions while a smaller ionic component is also active.
Perhaps influenced by the above theories, all electrode plasma torches (i.e. transferred arcs and d.c. arcs but not induction plasmas which are electrodeless torches) use either copper (a field emitter) or tungsten, carbon and molybdenum (thermionic emitters) for their cathode. Alloying elements, such as silver for copper and thoria for tungsten, are commonly used in concentrations up to 2%. These elements reduce the cathodes work function (a measure of the material's ability to emit electrons) thus allowing the cathode to operate at lower temperatures and minimum erosion rates.
Carbon electrodes do not include alloying elements. Thus, they have a relatively high work function (5.0 eV), exhibit high erosion rates and are referred to as a consumable electrode. The work function of tungsten is much lower at around 4.5 eV. However, pure tungsten would still erode rapidly in a plasma torch. The addition of 1% thoria can reduce the cathodic work function to below 3.0 eV allowing for a much more stable operation. Thoriated tungsten is the preferred cathode for thermionic emitting plasma torches.
The electrodes developed thus far provide low erosion rates and stable operations within a limited operating range. Most operate well at currents below 5,000 A and inert plasmagas. Copper alloys have been used successfully with oxygen as the plasmagas. Thoriated tungsten performs well in reducing plasmas (i.e. plasmas where H.sub.2, CH.sub.4, or NH.sub.4 are used in the plasmagas). However, no electrode to date has been successful in producing a stable operation in highly reactive plasmas, especially halogens.
Metal halide gases (such as TiCl.sub.4, NbCl.sub.5, etc.) are extremely corrosive at high temperatures. Thus, when such gases are used as the plasmagas in a torch, they react extensively with the cathode material. These reactions are deleterious to the plasma stability not only due to the mass loss occurring at the electrode but also due to the production of reduced metals (such as Ti and Nb) which blanket the electrode, increase its work function and ultimately suffocate the electron emission process.
A stable electrode for plasma torches operating on metal halide plasmagas must not react with such gases even at the extremely high temperatures characteristic of the cathode's surface (around 3,700.degree. C.). It must also posses a low work function, high melting and vaporizing temperatures, good thermal and electrical conductivity, and high resistance to thermal shock.
Tantalum carbide is a refractory material whose melting and boiling temperatures are 3,850.degree. C. and 5,470.degree. C. respectively. It also possesses an extremely low thermionic work function of around 3.8 eV. Compared to most ceramics, it is an excellent conductor with a room-temperature electrical resistivity of only 25 microhm-cm and a thermal conductivity of 21 W/(m.K). Finally, it is extremely resistant to chemical attack by the chlorides even at high temperatures.
Despite all the excellent properties of tantalum carbide, its performance as an electrode is rather unsatisfactory. It is highly susceptible to shattering upon arc ignition due to thermal shock. During operation, its thermal conductivity is too low to dissipate the enormous amount of energy absorbed at the electrodes, resulting in local melting. Due to its high melting temperature, it is a difficult material to sinter to high volume fraction %. Volume fraction % of only 50-65% were achieved by sintering for half hour at temperatures up to 2,000.degree. C. The volume fraction % can be increased to 75-80% by sintering for half hour at temperatures over 2,400.degree. C. The material at low volume fraction % has poor strength and significantly reduced electrical and thermal conductivities, further contributing to the shattering and melting observed upon arc ignition. Finally, the material is very hard and brittle making it very difficult to shape into useful electrodes.